In the operation of a converter having a voltage source, system perturbations (line-conducted interference voltages) are inevitable. In this context, the spectrum of interference voltages can be subdivided into four frequency ranges:
______________________________________ 1. 0 Hz . . . 2 kHz 2. 2 kHz . . . 9 kHz 3. 9 kHz . . . 150 kHz 4. 150 kHz . . . 30 MHZ ______________________________________
For the fourth frequency range, there are binding limiting values (limiting value class A/B) which, in the context of the Council of Europe Declaration of Conformity, must be observed with regard to electromagnetic compatibility (EC). Limiting values for the second frequency range are already in the planning stage.
The requirements of power stations, in particular of VDEW (Association of German Power Stations), with regard to improving the power factor, relate to the first frequency range and cannot be met using a three-phase bridge circuit such as the conventional B6-bridge (see Brosch: "Modern Power Converter Drives," first edition, Wurzburg, Vogel Printing House, 1989, p. 91ff). For meeting these requirements, a converter having sinusoidal current input is necessary.
Thus, the spectrum of interferences is shifted to the second and the third frequency range. In this context, one advantage is that in this frequency range the interferences can be filtered out at reasonable expense.
However, for the operation of a converter having a regulated voltage source, the second and third frequency ranges are of decisive importance even apart from the question of limiting values. In this frequency range, the interference voltage is very high and, as a result of the offending noise, it can lead to interference with other units being operated on the same power system.
Conventional power-line filters have been developed for the fourth frequency range and for the operation of converters using B6-bridge circuits (with an unregulated voltage source). Nevertheless, the frequency ranges below that are still not taken into account.
In the operation of converters having a regulated voltage source, in a filter of this type, resonances can lead to a magnification of the interference voltages and thus to a worsening of the problems described.